Method for controlling wall thickness in extruded tubular synthetic-resin blank

ABSTRACT

A mold has a core surrounded by an outer sleeve which defines with the core an annular axially extending passage. An adjustment element in the form of a ring or sleeve is displaceable in this passage so as to vary the radial dimension of a tubular blank that is extruded from the outlet end of the passage. The adjustment ring or sleeve may be elastically deformable by means of hydraulic cylinders or screws so as to create the desired thickness at the desired location, and may even be displaced during operation of the apparatus so as to vary the thickness of the blank from one region to another in the direction in which it is extruded.

This is a continuation of application Ser. No. 854,748, filed Nov. 25,1977, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method of and an apparatus forcontrolling the wall thickness in an extruded synthetic-resin blank.More particularly this invention concerns the production of such a blankwhich is thereafter to be blow molded into a container such as a bottle.

In the production of hollow bodies such as containers, tubes, and thelike it is standard practice to transform a solid and plasticsynthetic-resin strand first into a tubular and plastic synthetic-resinstrand. Thereafter this tubular strand is formed in a mold or similarequipment into the desired article.

It is almost always necessary that the desired article has a uniformwall thickness, that is that it be of substantially the same wallthickness throughout.

Typically the solid strand is fed to a shaping nozzle having a centralmandrel which transforms the solid strand into a tubular strand thatpasses through an annular passage which may contain several large-volumecompartments to an annular outlet. Pistons may be provided in thesecompartments or the synthetic-resin material may simply be forcedthrough the passage continuously so that a tubular synthetic-resin blankissues from the annular outlet of the shaping nozzle. The pin or mandrelthat transforms the solid strand into a tubular strand is typicallyconstituted as the core or central part of the nozzle and is supportedon the outer part of the nozzle by means of struts or webs. Theselast-mentioned struts or webs must inherently extend radially throughthe passage along which the synthetic-resin material flows so that theflow must pass around these struts or webs. After flowing around suchformations the material reunites. To this end the core may be made ofoppositely tapered shape so as to maintain a uniform flow cross-sectionthroughout the passage. It is also possible to angularly offset axiallyspaced struts or webs in order to minimize the interruption of flow forthe tubular strand.

It has also been suggested in arrangements wherein the solid strand isintroduced radially into the shaping nozzle to subdivide this strandinto a pair of branches which each are eventually fed at the outlet sideof the nozzle to a respective semi-cylindrical passage. To this end thedistribution chamber inside the shaping nozzle is heart-shaped and thepoints of such heart-shaped chambers overlap each other. Various otherformations are used to insure uniform flow and it is also known toprofile the core of such a mold. Such arrangements can be seen in U.S.Pat. Nos. 3,114,932; 3,217,360 and 3,309,443 whose entire disclosuresare herewith incorporated by reference.

Further known arrangements can be seen in Canadian Pat. No. 788,896,French Pat. No. 1,279,158, German Pat. Nos. 1,236,173 and 1,704,850 aswell as in German Utility Model No. 1,984,772.

All such arrangements have nonetheless shown themselves not able toproduce finish products after blow molding having the exactly requiredwall thicknesses. In particular in the production of bottles or the likeof synthetic-resin material the desired thickness at and adjacent thebase and neck of the bottle is almost impossible to obtain with theknown methods. Furthermore when the composition of the material beingmolded changes it is normally necessary to replace the entire nozzleassembly in order to insure proper molding of the new material.

Furthermore due to the flow characteristics of the synthetic-resinmaterial it is frequently necessary to decenter the core of the mold.The result of this is that the plastic synthetic-resin material flowsmost rapidly in those regions where it is the hottest or where thepressure drop is the greatest. It is necessary to reduce the mold gap orpassage width at exactly this location in order to obtain uniform flowthroughout the shaping nozzle. When this is done the blank produced bysuch a mold is of irregular thickness and hottest in the thinnestregion, so that it becomes very difficult to form many types of articleswith such a blank. When such a virtually randomly irregular blank isused to produce a container or the like the uncontrollableirregularities in the blank are often multiplied during the blowmolding.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved method of and apparatus for making a tubular synthetic-resinblank.

Another object is the provision of an improved method and apparatus formaking such a blank which is ideally suited for blow molding and whereinit is possible to obtain the exactly desired wall thickness at anylocation in the finished blow-molded article.

These objects are attained according to the present invention in amethod and apparatus which take into account not only the fact thatcertain regions are stretched more than other regions in the blowmolding, but that certain regions are more stretchable than others. Thusaccording to this invention the high-stretch regions of the tubularstrand produced from the solid strand are given a relatively largethickness and the low-stretch regions are given a relatively smallthickness. It is noted in this context that the high-stretch regionsinclude those regions where the flow was interrupted so that the tubularblank is particularly susceptible to stretching.

It is also a feature of the instant invention to vary the thickness ofthe workpiece along the tubular blank, in particular in the areas thatwill eventually be made into the neck and bottom of a container. This isachieved by changing the cross-sectional shape and/or radial dimensionof the annular passage as the material is being extruded through it.

The high-stretch and low-stretch regions of the tubular strand are giventheir respective thicknesses in accordance with this invention in twoseparate stages. In one stage, preferably at an upstream location, acontinuous welt and continuous thinned regions are formed. At adownstream location a variable formation is used in the shaping nozzlefor forming axially or longitudinally offset thickened and thinnedregions.

According to further features of this invention the shaping nozzle has acore member which defines an axis that corresponds to the direction orpath along which the material is extruded. An outer member defines withthis core member an annular passage having an annular outlet at theaxial end, normally lower, surface of the shaping nozzle. The passagedefined between the core and outer members is of generally uniformradial dimension measured at any plane perpendicular to the axis, theradial dimension being, of course, variable in the axial direction.Inlet means is provided in the shaping nozzle for transforming the solidstrand coming from an extruder or the like in an axial or radialdirection into a tubular strand. Finally an adjustment element isprovided in the passage between the members so as to define therein aregion of variable radial dimension which is different from thepredetermined radial dimension of the passage. Means is associated withthis adjustment element for displacing it relative to the members andthereby changing the variable dimension. The core member may be mountedwithin the outer member in the manner shown in my copending applicationSer. No. 804,378 filed June 7, 1977, now U.S. Pat. No. 4,120,633 theentire disclosure of which is herewith incorporated by reference.

The adjustment element may be a highly elastic ring or collar that isaxially displaceable or radially deformable. Alternately it is possiblefor the adjustment element to be a rigid member which is eccentricallyor axially displaceable and has a shape corresponding to that of thedesired tubular blank.

In accordance with the instant invention it is possible to use a pair ofaxially elongated elastic rings flanking the passage at the outlet end.The one ring is displaced by means of screws or the like into thedesired position and the other ring may be periodically deflected so asto obtain the desired thickness and thinness along the workpiece as itis extruded.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the stretching to which a tubular blankis normally subjected;

FIGS. 2 and 3 are diagrams illustrating the prior-art systems;

FIGS. 4 and 5 are diagrams illustrating the system according to theinstant invention;

FIG. 6 is an axial section through a portion of a nozzle according tothis invention;

FIG. 7 is an axial section through a second embodiment of the nozzleaccording to this invention;

FIG. 8 is a large-scale sectional view through a third arrangementaccording to this invention;

FIG. 9 is a large-scale sectional view through a variation on the thirdembodiment of FIG. 8;

FIG. 10 is an axial section through a fourth embodiment of the shapingnozzle in accordance with this invention;

FIG. 11 is a section taken along the line XI--XI of FIG. 10;

FIG. 12 is an axial section through a fifth embodiment of the nozzleaccording to this invention;

FIG. 13 is a section taken along line XIII--XIII of FIG. 12;

FIG. 14 is an axial section through a sixth embodiment of the apparatusaccording to this invention;

FIG. 15 is a section taken along line XV--XV of FIG. 14;

FIG. 16 is an axial section through a seventh embodiment of the nozzlein accordance with the instant invention;

FIG. 17 is a section taken along line XVII--XVII of FIG. 16;

FIG. 18 is an axial section through an eighth embodiment of the nozzleaccording to this invention;

FIG. 19 is a section taken along line XIX--XIX of FIG. 18;

FIG. 20 is an axial section through a ninth embodiment of thearrangement according to this invention;

FIG. 21 is a horizontal section taken along line XXI--XXI of FIG. 6;

FIG. 22 is an axial section through a portion of a tenth nozzle inaccordance with this invention; and

FIGS. 23, 24 and 25 are axial sections through eleventh, twelfth andthirteenth arrangements according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a curve 1 basically formed as a pair of adjacent halves ofa sine wave which represent the elongation to which a tubular blank isshown with the ordinate direction 2 indicating the amount of stretch andthe abscissa direction 3 indicating the area along the circumference ofa complete tubular blank. Typically the bottom of such a blank ispinched together along a plane perpendicular to its path of displacementand thereafter the blank is inflated. Most stretch occurs at thoseregions in the sides at the base of the blank 90° offset from the planealong which the mold closes and where the blank is pinched together.

In the prior art as shown in FIG. 2 the blank prior to such deformationhas a regular thickness shown at 4 so that when stretched as shown inFIG. 1 the finished product has as shown in FIG. 3 a thickness indicatedby the curve 5 which shows that at the regions of most stretching thearticle is the thinnest and at the region of least stretching thearticle is thickest.

According to this invention the workpiece is formed as shown by curve 6of FIG. 4 so as to be much thicker at those regions to be subjected tothe most stretching or which are most stretchable. The result, afterstretching as shown in FIG. 1, is a workpiece as shown in FIG. 5 havinga uniform thickness 7. This thickness 7 corresponds to the minimumthickness of the tubular blank at the region where it is subject to theleast stretching, that is the two edges lying directly on the plane atwhich the sides of the blank are pinched together.

A first embodiment of a shaping nozzle according to the instantinvention is shown in FIGS. 6 and 21. This nozzle has a core member 12surrounded by an outer two-part sleeve member 11 which defines with thecore member 12 an annular passage 10 starting at its upper end at adistribution chamber 9 adjacent a radial inlet 8 and extending past anadjustable section-changing mechanism 13 which is described below. Atits lower region the passage 10 becomes a passage 14 which terminates atan outwardly flared outlet portion 15 defined by an outwardly anddownwardly flared central pin or mandrel 17 which is axiallydisplaceable on a rod 16 for adjustment of its radial dimension.

An outer fixed ring 18 is associated with an inner highly elastic ring19 constituting an adjustment element. This ring 19 may be formed ofsemi-circular shape as shown at 54 in FIG. 21 and have an inner surface54a which defines a crescent-shaped compartment with an inner sleeve 23.A wedge 53 is engageable between the element 54 and the outer fixed ring18. Tangential displacement of this wedge 53 can therefore determine theshape of the passage 14 at the respective segment thereof.

FIG. 7 shows an arrangement similar to FIG. 6, but wherein an axiallydisplaceable piston 20 carried on a piston rod 21 defines a passageportion 22 of variable size. Reciprocation of this piston 20 allows aportion of material in the passage or compartment 22 to be pumpedtherefrom.

FIG. 8 shows an adjustment mechanism 13 which employs a highly elasticring 23 having a cylindrical inner surface that defines a passageportion 25. Screws 24 engage radially inwardly against the outerperiphery of the elastic ring 23 and are threaded into a holder ring 26positioned by means of screws 27 on the outer member 11. Another highlyelastic ring 28 of triangular section is provided above the ring 25 andrests thereon. This ring 28 defines an inner passage 30 that is acontinuation of the passage 10 and of the passage 25 and an outerpassage 31 that opens into a chamber 32 above and a chamber 33 behindthe ring 23. Screws 29 serve to radially position the ring 28.Furthermore the lower end of the chamber 33 is connected via a bleed orbranched passage or conduit 34 to a radially extending branch conduit orpassage 35 which is provided with a screw 36 that can act as a valve orflow controller in this passage 35. Thus during operation when thepassage 10 is pressurized with molten synthetic-resin plastic materialthis material will pressurize chambers to both sides of both of therings 28 and 23, therefore making their outward deflection by thepressure of this material less likely. In addition some flow is possibleby means of the branch conduit constituted by the chambers 31-35 so thatthe material will not solidify behind the rings 28 and 23. The materialthat exits past the valve screw 36 can be returned to the extruder andthen to the passage 10.

It is also possible to replace the screws 24 of FIG. 8 with tensionscrews 37 as shown in FIG. 9 which are screwed into sleeves welded tothe outer periphery of the sleeve 23 and which have heads that bearradially inwardly against the holder 26.

FIGS. 10 and 11 show another arrangement wherein the adjustment elementis carried on an axially displaceable piston or tube 38 and is formed asa collar 38a thereon having a profiled generally frustoconical surface38b that defines a portion of the wall of the passage through thenozzle. As best shown in FIG. 11 this collar or extension 38a is not ofregular thickness so that axial displacement of the element 38 willchange the thickness and shape of the passage through the nozzle.

In FIG. 12 the member 40 forming part of the passage is itself notaxially displaceable, but is radially deflectable and is engaged by afrustoconical surface of a member 39 carried on the rod 16. Thisfrustoconical skirt can therefore be deformed into the shape shown inFIG. 13, which is identical to the shape of FIG. 11.

In FIGS. 14 and 15 the passage 15 is formed at region 41 by a radiallydisplaceable eccentric element 42 having an inner periphery 42a ofnon-cylindrical shape or a shape at least not congruent to the coremember 17. Thus radial displacement of this eccentric element 42 bymeans of an assembly such as shown in FIG. 8 can adjust the shape of thepassage at 41.

Another such arrangement is shown in FIGS. 16 and 17 wherein an outerdisk member 44 has a frustoconical surface that bears on a radiallyinwardly deflectable skirt 43 at the outlet portion 15 of the passagethrough the nozzle. Thus axial displacement can deflect this skirt 43inwardly and outwardly to change the shape of the passage at the outlet15. The skirt 43 and the corresponding surface of the element 44 are ofnon-cylindrical shape.

The arrangement of FIGS. 18 and 19 has a triangular-section ring 45constituted as the lower end of an axially displaceable member 45a. Thisring 45 has a surface that therefore forms a restricted portion 46 ofthe passage through the nozzle. In addition above the ring 45 theelement 45a is formed with a throughgoing hole 48 that forms passages 47behind the ring 45. Thus the pressure in the region 46 will not preventsliding of the ring 45 on the core element.

In FIG. 20 a main distribution passage 49 communicates past theadjustment device 13 with a passage 51 and is provided with a branchpassage 50, 52 which extends past this adjustment device 13. The twopassages are united at their lower end above an axially displaceablesurface 52b of an element 52a. Such axial or vertical displacement ofthe element 52a opens or closes the lower end of the passage 51 andtherefore not only controls flow therefrom but can control the directionof displacement of the blank as it leaves the nozzle.

With the arrangements of FIGS. 19 and 20 it is possible relativelyeasily by axial displacement of the adjustment member to form thin andthick regions along the blank as it is extruded. Thus it is possible tothicken the blank and the region will form both the base and neck of abottle or the like, otherwise maintaining the tubular blank relativelythin.

The arrangement of FIG. 22 can be used in the systems of FIGS. 8 or 9.It has a highly elastically deformable adjustment ring 55 having anupper end 55a seated in the ring 28 and a lower end 55b seated in thefixed portion 18. A holder 56 can seat screws that serve to adjust it inthe manner shown in FIG. 8 and in FIG. 9. This adjustment member 55 iscompletely cylindrical and is formed of steel. Obviously the chamberbehind it may also be pressurized as shown in FIGS. 8 and 9 so that thepressure inside it does not deflect it outwardly.

The arrangement of FIG. 23 has an axial inlet 57 which communicates withaxially offset distribution passages 58 and 59 that join at a passage 60of relatively short axial width. Part of the wall of this passage 60 isdefined by a disk 62 which can be displaced axially by means of a screw63 and which lies underneath a counter-pressure chamber 61 communicatingwith the passage 58. In addition a radially displaceable generallytriangular-section ring 65 defines another portion 64 of the passagethrough the nozzle and is displaceable by means of a screw 66. Thescrews 63 and 66 are readily accessible from outside the machine andeither of these screws can be replaced by a hydraulic or pneumaticcylinder controlled from a central computer or controller so asperiodically to vary the radial or axial dimension of the passage at therespective location.

In FIG. 24 an inner ring 67 has an upper or upstream and 67a ofrectangular section received in a corresponding groove in the core 76.Its downstream or lower end 67b is flared radially outwardly and isengaged by screws 75 projecting upwardly at an angle to the axis of thenozzle and perpendicular to the outwardly flared portion 67b. Thus it ispossible by adjustment of any of the screws 75 to control the width ofthe passage 72 at the outlet 70.

Radially outside this inner adjustment ring 67 is an outer adjustmentring 74 that forms the outer wall of the passage and which is engaged ateach of a plurality of segments by segmental pushers 79 each on thepiston rod 78 of a small hydraulic cylinder 77. Each of these cylinders77 is controlled by a master programmed controller which serves tocontrol the thickness of the tubular blank produced by the shapingnozzle and exiting from the outlet 70 as it is produced.

It is also possible to use only an outer sleeve or ring 68 as shown inFIG. 25 and having a triangular upper bead or end 68a received in acorresponding formation of the outer member 69. The lower end 68b isradially inwardly tapered and forms passage 73 terminating at the outlet71. Means such as shown at 77 in FIG. 24 may be provided for radiallydeflecting this ring 68.

The thermoplastic synthetic-resin material that is shaped by the nozzleaccording to this invention is typically fed from an extruder in more orless homogeneous condition. When this hot homogeneous material is fedradially into the nozzle it must travel longer in some regions than inothers so that it cools. Similarly when fed axially in some of thematerial is deflected around struts or webs which support the core ofthe nozzle so that once again the material will in part be cooler thanin other parts. With the nozzle according to this invention it ispossible to cancel out the effects of this variation in heating, whichresults in different consistencies and stretchabilities of thesynthetic-resin material. Furthermore feeding the material in partthrough a branch conduit according to this invention further reducesirregularities from one portion to another.

Furthermore according to this invention it is possible to form articlesadapted for a particular use with the most efficient possible use ofsynthetic-resin material. More particularly those regions of an articlesubjected to very little wear, or which need not be very strong, can bemade relatively thin whereas other areas can be made much thicker.Furthermore such variation from one region to another can be not onlyangularly about the object being blow molded, but can be axially fromone location to another on the article, the axis here of course beingthe axis along which the material is extruded.

It is noted in this context that any of the features of any of theembodiments can be applied by any of the other embodiments. Thus, forexample, the adjustment cylinder of FIG. 24 could be used in thearrangements of FIGS. 8 or 9. Similarly the use of two sleeves such asshown in FIG. 24 could be applied to any of the other arrangements. Allsuch combinations are considered to lie within the scope of the instantinvention.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofmolding systems differing from the types described above.

While the invention has been illustrated and described as embodied in ashaping system for blow molding, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:
 1. In a method of producing containers of thermoplastic material wherein a solid strand of plastic synthetic resin is first formed into a tubular plastic strand and subsequently formed into a container having a closed bottom by squeezing together the tubular strand and blow molding the container, the steps of periodically deforming said tubular strand to vary its thickness at axially spaced portions by passing said tubular strand through an axially stationary ring nozzle defining an annular passage of non-uniform width for the flow of the plastic synthetic resin therethrough; and periodically adjusting the cross section of said annular passage to thereby asymmetrically deform said tubular strand over a portion of its circumference to impart to a high stretch region of said tubular strand a larger wall thickness than to a low stretch region thereof, so that the wall thickness of the strand portions forming the corners of the container is reduced in such a manner that after blow molding the container the latter will have a substantially uniform wall thickness.
 2. The method as defined in claim 1, wherein said annular passage of said ring nozzle is defined between a stationary mandrel of solid material and an elastic ring surrounding the mandrel spaced therefrom and wherein the step of periodically adjusting the cross section of said annular passage comprises the step of deforming a portion of said elastic ring in radial direction.
 3. The method as defined in claim 1, wherein said non-uniform width of said annular passage of said ring nozzle is adjustable in radial direction.
 4. The method as defined in claim 1, wherein said asymmetrical deforming of the wall thickness of said tubular strand is carried out in two axially displaced zones.
 5. The method as defined in claim 4, wherein in one of said axially displaced zones a constant asymmetrical deformation of the wall thickness is carried out by passing the annular strand through a first ring nozzle having an annular passage of non-uniform width, and wherein in the other of the axially displaced zones the annular strand is passed through a second ring nozzle having an annular passage of non-uniform width and including the step of periodically changing the cross section of the annular passage through the other nozzle. 